Fuel cells produce energy by combining hydrogen and oxygen to produce water and an end product. In a Polymer-Electrolyte-Membrane (PEM) fuel cell, a polymer membrane serves as the electrolyte between a cathode and an anode. In the PEM fuel cell, multiple fuel cells are frequently stacked in series to form a fuel cell stack. In the fuel cell stack, one side of a flow field plate serves as the anode for one fuel cell while the opposite side of the flow field plate serves as the cathode for an adjacent fuel cell. Because each flow field plate serves as both an anode and a cathode, the flow field plate is also known as a bipolar plate.
Conventionally, fuel cell manufacturers have used Poco graphite bipolar plates, which are electrically-conducting and resistant to corrosion in the fuel cell environment. However, graphite plates are brittle, and therefore, difficult to machine. This adds to the cost of the bipolar plates and volumetric power density of the fuel cell stack. While the use of metal bipolar plates is advantageous, metals such as titanium and 316L stainless steel, which can be easily machined, are easily attacked by fluoride ions in a fuel cell environment.
While 316L stainless steel exhibits a fair corrosion resistance to fluoride ions, the corrosion rate increases with increases in the fluoride ion leach out rate. Furthermore, it is well known that the fluoride corrosion resistance increases with the increase in the molybdenum content of the stainless steel alloy. This problem can be mitigated somewhat by removing the hydrogen fluoride from the fuel cell environment or by using higher grades of stainless steel which are more resistant to corrosion by fluoride ions than 316L stainless steel. However, the use of higher grades of stainless steel for the bipolar plate tends to increase the cost of the bipolar plate. Furthermore, incorporating higher-grade stainless steel into bipolar plates having the required thickness adds a significant cost to the bipolar plates that can exceed the cost of the stainless steel itself.
Various methods are known for increasing the corrosion resistance of a corrosion-susceptible substrate. For example, US20030228512 A1 discloses a method of improving the contact resistance of the surface of a stainless steel substrate while maintaining optimum corrosion resistance of the substrate by depositing a gold coating on the substrate. US20040091768 A1 discloses a method of increasing the corrosion resistance of a substrate by providing a polymeric conductive coating on the substrate. U.S. Pat. No. 6,372,376 B1 discloses a method of increasing the corrosion resistance of a substrate by providing an electrically-conductive, corrosion-resistant polymer containing a plurality of electrically conductive, corrosion-resistant filler particles on the substrate.
Coating the surface of a lower grade stainless steel bipolar plate, such as a 304 L or 316L stainless steel bipolar plate, for example, with a thin layer of high-grade stainless steel or alloy using a kinetic or cold spray process imparts a high degree of fluoride ion corrosion resistance to the bipolar plate while maintaining the cost of the bipolar plate within acceptable levels. Furthermore, a kinetic or cold spray process can be used to deposit a corrosion-resistant layer having a thickness of up to 25 microns or more.